1. Field of the Invention
The present invention relates to a knock determining apparatus, which is suitably used in, for example, control of an ignition timing of an internal combustion engine.
2. Description of the Related Art
In a spark ignition internal combustion engine, a higher output can be obtained as the compression ratio of the air-fuel mixture is higher. When the compression ratio is high, however, so-called knock often occurs in which, after the air-fuel mixture introduced into a cylinder is ignited by a spark, an unburned gas explodes in advance of a flame propagating in the cylinder. When knock occurs in an internal combustion engine, pressures applied to the pistons become uneven to make the operation of the engine irregular, and the temperature of the engine is raised to lower the thermal efficiency, with the result that the engine power is lowered. In an internal combustion engine, therefore, it is determined whether knock occurs or not, and, if knock occurs, a control of adjusting, for example, the ignition timing is conducted to suppress occurrence of knock.
In an internal combustion engine, a method is employed in which a knock sensor configured by a piezoelectric device or the like is disposed in each of cylinders and occurrence of knock is determined on the basis of the detection output of the knock sensor. When the spectrum of the detection output of a knock sensor is sampled, it is seen that the output level when knock occurs is larger than that when knock does not occur. The difference between the output level when knock occurs, and that when knock does not occur is not uniform over the entire frequency range of the detection output of the knock sensor, but conspicuously appears at a specific frequency. The frequency at which the output level difference conspicuously appears is called the resonance frequency.
A knock sensor in which the detection range is limited to the resonance frequency band where the output level difference is large is a resonance knock sensor. In a resonance knock sensor, the specific resonance frequency is previously decided by setting the sensor itself. Specifically, the resonance frequency is set to a frequency band in which it is estimated that the output level difference is large in a substantially common manner while sacrificing the characteristics and the S/N ratio that depend on cylinders of an internal combustion engine and conditions such as the rotational speed.
FIG. 8 diagrammatically shows determination of occurrence of knock in the related art. In the determination of occurrence of knock in the related art, if the detection output of a knock sensor reaches one or more times the determination level, it is determined that knock occurs. In an internal combustion engine, in addition to vibrations due to knock, also vibrations due to the ignition timings of ignition plugs and seat timings of inlet/outlet and exhaust valves occur. Detection outputs of a knock sensor, which are produced vibrations due to a cause other than knock, are called background noises 1 and 2. When the frequency of a knock signal 3 which is the detection output of the knock sensor produced by vibrations due to knock is close to the frequencies of the background noises 1 and 2, also the background noises 1 and 2 are sometimes detected with being included together with the knock signal 3 in the resonance frequency band which is set in a resonance knock sensor.
In the case where both the knock signal 3 and the background noises 1 and 2 exist in the resonance frequency band, in the related art, discrimination of the knock signal 3 from the background noises 1 and 2 is enabled by raising the determination level from a first determination level 4 to a second determination level 5, and occurrence of knock is then determined.
In the technique in which the determination level is simply changed, when the intensities of the background noises 1 and 2 are so high that the intensity difference between the knock signal 3 and the background noises 1 and 2 is small, it is difficult to discriminate the knock signal 3 from the background noises 1 and 2. Hence, there is a problem in that occurrence of knock cannot be accurately determined.
In a related art example shown in FIG. 8, it is determined that knock occurs based on the detection output of the knock sensor, which reaches the second determination level 5 and is in the state where the output contains not only the knock signal 3 but also the background noise 1. Although not illustrated, there may be a case where it is determined that knock occurs, notwithstanding that the detection output, which reaches the determination level, contains only background noises.
Namely, even when it is difficult to discriminate the knock signal from the background noises and determination is actually impossible, if the detection output of the knock sensor reaches one or more times the determination level, it is always determined that knock occurs. Therefore, the related art example is not a knock determining method, which is always accurate.
One of reasons why the frequency of the knock signal and frequencies of background noises are close to one another to exist together in the resonance frequency band is that the frequency of the knock signal is varied by changes with time or the like in the operation of an internal combustion engine. In this case, in accordance with variation of the frequency of the knock signal, the resonance frequency band which is set in the knock sensor is corrected so as to be shifted, whereby the accuracy of detection of the knock signal in the detection output of the knock sensor can be improved.
The related art technique of shifting the resonance frequency band is disclosed in, for example, JP-A-Hei.8-177697. In the related art technique, a flat sensor in which the detecting frequency is not fixed to a specific frequency is used as a knock sensor, and the setting of the flat sensor itself can be changed depending on the kind of the internal combustion engine, the rotational speed, and the cylinder. Flat sensors can be set so as to have different resonance frequencies, respectively. When the frequency of the knock signal is varied by changes with time or the like in the operation of an internal combustion engine to be deviated from the resonance frequency set in one of the flat sensors, the standard deviation in the intensity distribution of the detection output of the knock sensor is calculated, and the calculated standard deviation is compared with a given value. If the standard deviation is smaller than the given value, it is determined that the resonance frequency is deviated, and the accuracy of knock detection is improved by correcting the resonance frequency set in the flat sensor. In the related art technique, since deviation of the resonance frequency is determined on the basis of a result of the calculation of the standard deviation in the intensity distribution of the detection output of the knock sensor, there is a problem in that the operation of detecting deviation of the resonance frequency is very complicated.
It is an object of the invention to provide a knock determining apparatus for an internal combustion engine in which reduction of erroneous determination of occurrence of knock, and determination of deviation of the resonance frequency are performed by a simple determining unit, and the deviation of the resonance frequency is corrected on the basis of a result of the determination, whereby the accuracy of knock detection can be improved.